The field of the invention is nuclear magnetic resonance imaging (MRI) methods and systems. More particularly, the invention relates to the compensation of fast spin echo pulse sequences to reduce image artifacts caused by eddy currents.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B.sub.0), the individual magnetic moments of the spins in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B.sub.1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, M.sub.z, may be rotated, or "tipped", into the x-y plane to produce a net transverse magnetic moment M.sub.t. A signal is emitted by the excited spins after the excitation signal B.sub.1 is terminated and this "MR" signal may be received and processed to form an image.
When utilizing these signals to produce images, magnetic field gradients (G.sub.x G.sub.y and G.sub.z) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image: using one of many well known reconstruction techniques.
Most MR scans currently used to produce medical images require many minutes to acquire the necessary data. The reduction of this scan time is an important consideration, since reduced scan time increases patient throughput, improves patient comfort, and improves image quality by reducing motion artifacts. There is a class of pulse sequences which have a very short repetition time (TR) and result in complete scans which can be conducted in seconds rather than minutes. One of these is the Rapid Acquisition Relaxation Enhanced (RARE) sequence which is described by J. Hennig et al in an article in Magnetic Resonance in Medicine 3,823-833 (1986) entitled "RARE Imaging: A Fast Imaging Method for Clinical MR." The RARE sequence is a fast spin echo sequence which utilizes RF refocused echoes generated from a Carr-Purcell-Meiboom-Gill sequence. Such fast spin echo ("FSE") scans are very susceptible to image artifacts caused by eddy currents induced by the rapidly changing magnetic field gradients. While gradient pulse compensation techniques are adequate for scans performed with most MRI pulse sequences, it has been discovered that observable image artifacts are produced by 10% to 20% of the MRI scanners performing FSE scans.